The present invention relates generally to devices combining a catalytic converter with a muffler in a space-efficient and functionally-effective manner, particularly for small internal combustion engines. While the devices as disclosed herein are particularly suitable for small internal combustion engines ranging from approximately two horsepower to twenty horsepower, the invention may be applied to engines of smaller or larger sizes as well.
Nearly every small internal combustion engine includes a muffler, and a wide variety of muffler designs have been proposed and employed in the prior art. Relevant in the particular context of the present invention are various muffler designs including spiral channels. Spiral channel mufflers advantageously provide a relatively long channel length in a relatively compact space, and have the potential for highly effective noise reduction.
Thus, and as an example, in the case of an internal combustion engine operating under load at, for example, 3000 R.P.M., the exhaust valve opens very quickly to release a surge of exhaust gas which is under substantial residual pressure. For example, the pressure of an exhaust gas surge may be as high as 2.7 atmospheres. In the exemplary case of a twelve horsepower engine, the release could be in excess of sixty cubic inches of atmospheric pressure gas. It is quite common knowledge that, if not muffled, such sudden releases of pressure produce considerable objectionable noises or compressional sound waves at various frequency components. In the case of a four-stroke cycle engine, the exhaust gas surges are produced once every other revolution. Thus, in the example of a four-stroke cycle engine operating at 3000 R.P.M., or fifty revolutions per second, twenty-five exhaust gas surges are produced per second.
In addition to objectionable noise, exhaust gas from an internal combustion engine generally includes a variety of constituents that are considered to be pollutants. Such exhaust gas constituents include unburned hydrocarbons and carbon monoxide (CO). Engine exhaust gas is typically treated by means of a catalytic converter which promotes more complete oxidation of the exhaust gas constituents, changing unburned hydrocarbons to carbon dioxide (CO.sub.2) and water, and changing carbon monoxide (CO) to carbon dioxide (CO.sub.2).
In order for such oxidation to effectively occur, in most cases it is necessary to introduce atmospheric air containing oxygen into the exhaust gas stream upstream of the catalytic converter. A variety of devices have been proposed and employed in the prior art for introducing atmospheric air into the exhaust gas stream upstream of a catalytic converter, including air pumps to inject air at positive pressure at a suitable point in the exhaust gas stream, and venturis arranged to draw in atmospheric air through an air nozzle. As is known, a venturi has a throat of reduced cross sectional area which forces an increase in velocity and a consequent drop in pressure.
From the standpoint of cost as a consideration, the venturi approach is preferable to employing an air pump. Thus, a venturi arrangement is generally less expensive to manufacture at the outset, and is generally not subject to periodic maintenance as there are no moving parts.
A problem, however, in the use of venturis is that it is difficult to maintain proper venturi operation with an input gas flow in the form of periodic surges or slugs of gas. If not otherwise prevented, typically as the surge pressure gas plug passes through, it causes backflow of exhaust gas through the intended atmospheric air intake.
More particularly, gas movement in time involves three main factors: difference in pressure between points of interest in a gas flow system, cross-sectional area of a gas flow channel, and kinetic energy of gas flow, including inertia involving density and velocity, whereby gas flow has a forceful tendency to continue velocity and direction. Friction and viscosity are inhibiting factors of gas motion.
When an engine is operating at high performance and its exhaust valve quickly opens, a slug of gas under quite high residual cylinder pressure starts accelerating down an exhaust system. Due to slug inertia, in addition to that of older gas remaining downstream in the exhaust system, some of the elevated pressure survives for quite a distance downstream.
In fact, this elevated pressure can exceed the drop in pressure in the venturi throat. Thus, as a result of exhaust pressure in the venturi throat exceeding atmospheric pressure, there is backflow through the nozzle which is supposed to be admitting air, such that exhaust gas puffs out, rather than in.
Moreover, under such conditions, not only does gas flow objectionably puff out through the intended atmospheric air inlets, but insufficient air is mixed with exhaust gas flow for proper operation of the catalytic converter.
One prior art approach to this problem is to employ a check valve which, being an additional and a moving part, introduces added initial cost and is subject to subsequent failure necessitating repair or replacement.